This invention relates generally to mass spectrometers, and more particularly to double focusing mass spectrometers.
Mass spectrometers have earned a respected reputation for their unique ability to identify and quantitate a wide variety of chemical elements and compounds, often present in only trace level concentrations in complex chemical mixtures. Operating in a vacuum chamber, mass spectrometers ionize and fragment sample molecules, and through use of appropriate control and data capture electronics, generate a histogram of fragment molecular weight versus relative abundance of each ionic species present in the sample.
One class of mass spectrometer called a magnetic sector instrument uses a magnetic field at right angles to the ion beam trajectory to separate ions based on their mass-to-charge ratios. Single focusing magnetic sector instruments perform only directional focusing, while double focusing magnetic sector instruments provide both direction and velocity or energy focusing of the ion beam, usually by using an additional electrostatic energy analyzer in tandem with the magnetic sector analyzer, to achieve significantly higher resolution. Traditionally, both single and double focusing mass spectrometers are bulky, typically weighing 100-1500 kg, and thus confined to analytical laboratories due to their large size, weight, high power consumption and need for frequent service by skilled operators. Furthermore, due to their complexity and relatively low unit production rates, they are traditionally hand-made, one at a time, at a relatively high unit cost.
For example, U.S. Pat. No. 3,984,682 to H. Matsuda and U.S. Pat. No. 4,054,796 to M. Naito, show that the size of double focusing mass spectrometers may be significantly reduced by arranging the electric sector energy analyzer within the magnet pole gap. In contrast to tandem electric and magnetic
For example, U.S. Pat. No. 3,984,682 to H. Matsuda and U.S. Pat. No. 4,054,796 to M. Naito, show that the size of double focusing mass spectrometers may be significantly reduced by arranging the electric sector energy analyzer within the magnet pole gap. In contrast to tandem electric and magnetic sector analyzers, each of which contributes to the total ion path length, one obvious advantage of superimposing the magnetic and electric fields is a smaller instrument with a shorter ion path length between the ionizer or source and the ion detector. However, this advantage, which reduces the number of ion-molecule collisions, creates new challenges that must be overcome in order to achieve the desired performance. Specifically, it is difficult to generate the required orthogonal electric and magnetic fields within a single small volume without causing serious electric field degradation effects that degrade performance. According to Matsuda and Naito, two cylindrical sector, coaxially-aligned electrodes connected to a voltage source generate a radial electric field used in the energy analyzer portion of a double focusing mass spectrometer. In order to accommodate the small magnet gap axial dimension, these concentric electrodes must have a low axial height-to-separation ratio in order to allow sufficient radial separation to permit ion transmission along the central orbit of the ion beam. The small axial height of these electrodes results in undesirable fringe field effects in the ion path and thus sub-optimal resolution. Further, auxiliary electrodes are added between the upper and lower edges of these cylindrical electrodes to adjust the electric field in the central ion orbit to improve transmission and resolution. While the use of auxiliary electrodes greatly improves the geometry of the electric field in the electric sector, the presence of the cylindrical electrodes ultimately limits the further reduction of the magnet pole gap.
In addition, traditional electromagnets used to generate a 1-2 Tesla magnetic field in a 2 to 10 cm magnet pole gap are prohibitively expensive and bulky compared to rare earth permanent magnets now commonly used in smaller instruments requiring a fixed or non-scanning magnetic field. Furthermore, even with the newest high-energy-product NdFeB magnets, it remains extremely difficult and costly to generate 1-2 Tesla in a magnet pole gap larger than 1 cm.
Further, known mass spectrometers are large in physical size requiring significant installation space, usually in a well-regulated operating environment where temperature, humidity, vibration and other conditions are tightly controlled. Known mass spectrometers use heavy construction materials favoring discrete, usually stainless steel, components including vacuum manifolds, flanges, valves and supporting structural elements. This necessitates low quantity manual construction of each instrument at a relatively high unit cost. Also, known mass spectrometers require high electrical power consumption to run vacuum pumps, heaters, air conditioners, water coolers, electronics and ancillary equipment. Also, the complex design of known mass spectrometers require full-time, specially-skilled operators to use the equipment and perform routine maintenance and repairs, often requiring delicate alignment of internal elements and an inventory of spare parts.
The relatively large physical dimensions of present mass spectrometers require a lower operating pressure than smaller instruments in which ions traverse a shorter path between the ion source and detector. The mean free path length of a molecule in a vacuum system is inversely proportional to pressure and can be approximated by xcex=0.005/P, where xcex is the mean-free path length in centimeters and P is the pressure in Torr. As a general design rule, vacuum pumps are employed that maintain the mean free path length to an order of magnitude longer than the actual ion flight path length. Accordingly, microscale instruments can operate at higher sample gas pressure and require smaller, less expensive vacuum pumps. Since vacuum pumps represent some of the highest cost components in conventional mass spectrometers, a significant cost reduction benefit results from smaller, less expensive vacuum pumps.
Additionally, known double focusing mass spectrometers with larger electrode separations require higher voltages to create the same electric field as in smaller, functionally equivalent instruments in which electrodes are closer together. For example, a voltage source of 1000 volts is required to produce an electric field of 10,000 volts/meter between planar electrodes 10 cm apart, while a voltage source of only 50 volts is required to produce the same electric field between electrodes 5 mm apart.
Still further, known double focusing mass spectrometers have traditionally been constructed primarily of stainless steel housings, bolts, valves, transfer lines and structural supports with an essentially unlimited lifetime. Such construction has generally limited these instruments to use in stationary operating environments, typically laboratories or industrial plants.
It would be desirable to provide mass spectrometer sensors that are relatively small, manufactured from light weight materials, and have low electrical power requirements. Further, it would be desirable to provide mass spectrometer sensors that can be operated continuously without requiring full-time operators.
A mass sensor in accordance with an exemplary embodiment of the present invention, includes a magnet assembly and a mass analyzer. The mass analyzer includes a housing having a cavity therein. The housing is formed from two end plates and a center portion positioned between the plates. The mass analyzer further includes an ionizer, a double focusing mass spectrometer having superimposed orthogonal magnetic and electric fields, and an ion detector located in the housing cavity. The housing is formed from any suitable material, for example ceramic.
The double focusing mass spectrometer includes an electric sector energy analyzer having a film resistor deposited on an inside surface of each end. The film resistors are essentially concentric and congruent and have a circular arc shape and a radial width of at least five times the axial separation of the film resistors. Boundary electrodes are positioned adjacent each curved edge of each film resistor. The boundary electrodes are connected to a variable electrical voltage source so that the film resistors and the boundary electrodes combine to form a radial outward directed electric field.
The ionizer includes a filament and an anode located in a chamber formed in the outer wall of the center portion of the housing. The ionization chamber includes a slit opening into the housing cavity. At least one ion extraction electrode and at least one ion focusing electrode are located proximate the ionization chamber slit. The ion extraction electrodes and focusing electrodes are formed by photolithographically deposited metal strips on the inside surfaces of the end plates. The metal strips are positioned substantially parallel to one another with the metal strips deposited on one end plate aligned with a corresponding strip on the other end plate to form an extraction electrode or a focusing electrode. Further, an object slit electrode is located between the focusing electrodes and the electric sector energy analyzer.
The ion detector is one of a dynode electron multiplier, a continuous dynode electron multiplier, a microchannel plate detector, a microsphere detector, a charge coupled array or a magnetic electron multiplier. In an exemplary embodiment, the ion detector is a microchannel plate detector located in a chamber in the outer wall of the housing center portion. The detector chamber includes a slit opening into the housing cavity. An image slit electrode is deposited on the outer housing wall in the detector chamber slit opening.
The mass analyzer further includes a non-evaporable getter mounted inside the housing cavity. The non-evaporable getter is formed from a Zrxe2x80x94Vxe2x80x94Fe film deposited on a metal substrate. Also, the mass analyzer includes external electrical contacts arranged to form a multi-layer printed circuit card that is installable in a circuit card edge connector.
The magnet assembly of the mass sensor includes a ferromagnetic yoke, a first magnet pole element, and a second magnet pole element. The ferromagnetic yoke has a substantially C-shaped cross-section. The first and said second magnet pole elements are positioned with substantially parallel proximate faces separated by a gap sized to receive the mass analyzer. The mass analyzer is positioned in the gap between the first and the second magnet pole elements so that the ionizer and the ion detector are not proximate a magnetic field formed by the magnet pole elements.
The above described mass sensor provides for reduced physical dimensions to take advantage of smaller vacuum pumps allowing higher sample operating pressures and lower operation voltages, and eliminating the need for a well-regulated temperature, humidity and vibration environment. Also, the above described mass sensor provides significantly reduced sensor weight by eliminating inessential packaging components such as stainless steel vacuum manifolds and flanges and other discrete device elements in favor of newer alternative materials and an integrated design that exploits the use of photolithographic deposition of distributed electrical elements on substrate materials such as alumina or other ceramics which simplifies and reduces the number of manufacturing steps required in the fabrication process, allowing a higher degree of automation geared to high volume production and a lower cost per unit. Further, the above described mass sensor provides for reduced operating energy consumption by using smaller vacuum pumps, such as ion pumps, non-evaporable getters, liquid diffusion pumps and miniature mechanical pumps, and by lowering operating voltages and currents to allow operation from smaller energy sources such as automobile batteries and photovoltaic cells facilitating increased portability and deployment in remote locations. Still further, the above described mass sensor provides for reduced maintenance time and expenses by eliminating access to internal device elements, thus favoring the simple replacement by minimally skilled personnel of a single, disposable integrated mass sensor module in nearly all applications.